Onychomycosis, a fungal disease of the nail unit caused by yeasts, dermatophytes, or other molds, accounts for approximately 50% of all nail disorders in humans. In about 80% of onychomycosis cases, the toenails are infected, whereas in the remaining 20%, the fingernails are infected. The symptoms of this disease include split, thickened, hardened, and rough nail plates.
Another common disorder of nails is nail psoriasis, which affects up to 50% of patients with psoriasis. Characteristic nail psoriasis symptoms include pitting, which appears as punctuated or irregularly shaped depressions arranged on the surface of the body of the nail; discoloration of the nail bed; onycholysis or detachment of the body of the nail from the nail bed; subungual keratosis; or anomalies of the body of the nail. Other diseases and disorders involving the nails in humans and in other animals include onychia, onychocryptosis, onychodystrophy, onychogryposis, onycholysis, onychomadesis, onychophosis, onychoptosis, paronychia, koilonychia, subungual hematoma, and laminitis.
The nail plate is thick, hard, and dense, and represents a formidable barrier to drug penetration. Although nail material is similar in various ways to the stratum corneum of the skin, the nail is composed primarily of hard keratin which is highly disulfide-linked and is approximately 100-fold thicker than stratum corneum.
Various topical therapies have been suggested for treatment of nail disorders, such as onychomycosis. Nail lacquers, coating, polishes, enamels, and varnishes have been described. Bohn, U.S. Pat. No. 4,957,730, describes a nail varnish containing a water-insoluble film-forming substance and antimycotic compound. Ferro, U.S. Pat. No. 5,120,530, describes an antimycotic nail varnish containing amorolfine in quaternary ammonium acrylic copolymer. The water-insoluble film former is a copolymerizate of acrylic acid esters and methacrylic acid esters having a low content of quaternary ammonium groups. Bohn, U.S. Pat. No. 5,264,206, describes a nail lacquer with antimycotic activity, which contains an antimycotic agent and water-insoluble film formers including polyvinyl acetate, a copolymer of polyvinyl acetate and acrylic acid, copolymers of vinyl acetate and crotonic acid. Wohlrab, U.S. Pat. No. 5,346,692, describes a nail lacquer for treating onychomycosis, comprised of a film-forming agent, an antimycotically active substance, and urea, wherewith the antimycotic agent and urea are liberated from the lacquer when the lacquer is applied. A preferred formulation comprises cellulose derivatives as film former, clotrimazole as the antimycotic agent, dibutyl phthalate as a plasticizer, and a mixture of acetone and ethanol as solvent. Nimni, U.S. Pat. No. 5,487,776, describes a nail lacquer composition which forms a water permeable film containing griseofulvin when the organic solvent system evaporates, wherein a portion of the griseofulvin is in solution and a portion of griseofulvin is present as a colloidal suspension. Chaudhuri, U.S. Pat. No. 6,143,794, describes a topical formulation for the treatment of nail fungal infections that includes an antifungal, solvent, gelling agent, adhesion-promoting agent, film-forming agent, surfactant, and optionally a keratolytic agent. The adhesion-promoting agent was a hydroxy-terminated polyurethane such as polyolprepolymer-2. All of these patents and publications describe products applied to the nail that form a substantive nail coating or film containing a drug from which the drug is to penetrate into the nail. None of these methods has proven to be consistently effective in treating disorders of the nail such as onychomycosis.
Various topical therapies utilizing chemical compounds disclosed to enhance penetration through the nail have been described. Knowles, U.S. Pat. No. 5,652,256, describes the use of methyl acetate as a penetration enhancing compound in combination with naftifine or sulconazole and naftifine as a topical gel for fungal treatment of the nails. Sorenson, U.S. Pat. No. 5,972,317, discloses that a proteolytic enzyme such as papain, delivered by pads soaked in the enzyme solution, produces a more permeable nail. Sun, U.S. Pat. No. 6,231,875, describes acidified compositions of antifungals to enhance transport across nails and skin. Reeves, U.S. Pat. No. 6,391,879, describes the combination of an anti-fungal agent dissolved in an anhydrous blend of polyglycol and DMSO. Although these and other enhanced penetration formulations were reported to increase penetration through the nail, they have not been shown to be clinically effective in treating conditions of the nail, such as onychomycosis.
Because of the difficulty in obtaining clinically effective concentrations of medication to the nail bed by topical application of a pharmaceutical composition to the affected nail, nail disorders, such as onychomycosis, are typically treated with systemic medications or with topical medications following removal of the nail. Systemic treatment for onychomycosis and other nail disorders is often not satisfactory because therapy must be continued for long periods of time, often many weeks or months, and the medication has effects on tissues other than on the affected nail. Antifungal compounds, such as miconazole and ketoconazole, have been demonstrated to be effective in topically treating onychomycosis after nail removal. However, it is clear that removal of the nail is a measure than most individuals suffering from onychomycosis would prefer not to undergo if a less drastic therapeutic method would be efficacious.
Pitre, U.S. Patent Publication 2007/0041910, filed as U.S. patent application Ser. No. 11/432,410; and Mallard, U.S. Patent Publication 2006/0147383, filed as U.S. patent application Ser. No. 11/315,259, disclose that application of a pharmaceutical composition containing a vehicle, a volatile silicone, and a non-volatile oily phase, provides increased penetration of a pharmaceutically active compound when topically to skin or mucous membrane. This enhanced penetration is obtained without the use of glycols, such as propylene glycol, which are known to augment skin penetration of pharmaceutical compounds but which are also known to be irritating to skin. The formulations of Pitre and Mallard contain at least 25% w/w of a volatile silicone and, if formulated with an alcoholic vehicle, contain at least 15% of alcohol. All alcoholic compositions disclosed in Pitre and Mallard contain greater than 50% volatile silicone and the concentration of the volatile silicone is at least twice the concentration of the alcohol in the composition.
Pitre and Mallard do not disclose or suggest the use of such compositions for the treatment of diseases of a nail, such as onychomycosis. Moreover, studies have been conducted, including studies conducted in the laboratories of the present inventors, that show that the penetrating ability of an active agent from a composition into skin cannot be correlated to the penetrating ability of the active agent from the composition into or through a nail.
A significant need remains for a pharmaceutical composition that provides for enhanced penetration of a pharmaceutical agent contained within the composition into and through an intact nail. Such a composition would be valuable for topically treating conditions affecting the nail or nail bed, such as onychomycosis.